Unravelling the sediment transport mechanisms in an artificial lagoon

Multi-modal sand transport analysis at the hard flood defence of Maasvlakte 2

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Abstract

The port of Rotterdam was full and new mooring locations were required to host the largest ships in the world. And if there’s no space to expand on land, why not create land in the sea? That is exactly what The Netherlands did by creating Maasvlakte 2. The construction of Maasvlakte 2 started in 2008 and was officially completed on May 22nd , 2013. This research focused on the morphodynamics in the domain between the cube reef and cobble beach at the hard flood defence, which is named "the lagoon". Over the past 5 years a large sand volume propagated into the lagoon. The current sand volume (after 5 years) in the given domain is about four times larger than the predicted value by PUMA. The processes behind the sand layer formation were not fully understood.
This research started by creating a conceptual model with all processes that could contribute to the formation of the sand layer. All processes are divided into hydrodynamic or aeolian transport, whereby hydrodynamic transport is divided into overtopping over the cube reef and transport through the cube reef due to tidal currents. We performed field experiments at Maasvlakte 2 to measure the flow velocity in the lagoon and to measure the aeolian transport capacity. From a sediment budget analysis we observed a sand layer increase during winter periods. This increase is justified with the fact that sand transport towards the lagoon depends on extreme events, which happen mostly during winter periods. Based on the estimated transport volumes we concluded that the main mechanisms that contribute in the formation of the sand layer are overtopping over the cube reef and aeolian transport, whereby overtopping is the largest mechanism. Our estimates indicate a total maximum sand layer volume of 51,000m3 (whereby transport by overtopping = 25,000m3, tidal currents = 8,700m3 and aeolian transport = 17,300m3). Which leaves 20,000m3 of sand unaccounted for compared with our estimated 71,000 m3 volume of sand in the lagoon. The accuracy of each calculation is analysed in order to explain the missing volume compared with the total sand layer volume. Based on this analysis we concluded that only the uncertainty in the overtopping calculation can explain the missing volume of 20,000m3. High wind speeds and high waves will cause for sand transport towards the lagoon. Aeolian transport will always happen if there is a supply of granular material and atmospheric winds of sufficient strength. Moreover, the source for aeolian and hydrodynamic transport is the, southern located, soft protection. Since nourishments are necessary to secure the safety of the soft protection, the supply of sediment will remain. Sand is transported in northern direction along the hard protection by longshore transport. Currently, the water depth northwards of KP2700 is too deep to stir the sediment up and transport it into the lagoon by overtopping. Depending on the foreshore migration in the northern direction, the sand layer will also increase in northern direction.